Always talk to the parents first. Let them know they are invading the dog's space, and make them understand how the dog may react. If this does not work, send them a letter, keep a copy of it, and explain you will call the cops if they continue to torment the dogs.

If they refuse to listen, call the cops. Explain to them the steps you have taken, and explain, although the dog(s) may be friendly, they can react in a way that is not acceptable if they get tired of the tormenting. This way if the dog(s) do end up biting them, you have something in writing stating you had already approached the kids and the parents and they refused to abide.

This may or may not help, but at least you have something on your side should this happen. You might also want to contact animal control and see what they would advise.

Remember, continue to inform them and you show you tried everything you can to prevent an incident. If the kids jump the fence you have: animal harrassment and or trespassing. Easier to have prosecuted or at least a legitimate issue.

Good luck.